System and method for mounting electronic components onto flexible substrates

ABSTRACT

A system and method for reflowing solder to interconnect a plurality of electronic components ( 24 ) to a substrate ( 12 ) is disclosed. The system includes an oven for preheating the substrate ( 12 ) and the plurality of electronic components ( 24 ) disposed thereon, a supplemental heat source disposed in the oven for providing additional heat energy to reflow the solder ( 72 ), a pallet ( 14  ) for supporting the substrate ( 12 ), wherein the pallet ( 14 ) has at least one internal cavity ( 40 ), and a phase-transition material ( 42 ) disposed within the cavity ( 40 ) for absorbing heat from the pallet ( 14 ).

CROSS-REFERENCE TO PRIORITY APPLICATION

This application claims benefit of U.S. Provisional Application No.60/237,650 filed Oct. 2, 2000.

TECHNICAL FIELD

The present invention relates to an apparatus and method for reflowingsolder to electrically connect electronic components to a flexiblesubstrate having a low softening temperature.

BACKGROUND OF THE INVENTION

It is well known in the art to mount electronic components to rigid andflexible printed circuit boards. Typically, solder paste is applied toconductor pad regions on the rigid or flexible substrate. Components arethen placed with their terminals contacting the solder paste in the padregions. The substrate is then exposed to relatively high temperaturesto activate the solder paste which melts and then solidifies to bond andelectrically connect the components onto the substrate. The flexiblesubstrates are typically made from polyimide, which exhibits goodstability when exposed to high temperatures. Many film materials,including polyesters, have not been used satisfactorily for surfacemount components primarily because they exhibit inadequate heatresistance and dimensional stability when exposed to the temperaturesrequired for solder reflow.

A technique for mounting components onto flexible polyester substrateswith low softening temperatures is taught by Annable in U.S. Pat. No.5,898,992. The flexible substrate is fixed to a carrier support member.A cover is placed over the substrate. The cover has openingscorresponding to component locations and with the carrier forms acarrier assembly. Solder paste is applied to the conductor regions ofthe substrate having component pads. Electronic components are thenplaced on the substrate with their terminals in contact with the solderpaste. The carrier assembly is then pre-heated in a reflow oven to atemperature below the melting point of the solder paste. The assembly isthen subjected to a rapid rise in temperature utilizing a supplementalheat source such as a heated gas jet The cover shields the substratefrom the high reflow temperatures and minimizes distortion of theflexible substrate during reflow.

While the prior art teaching achieve their intended purpose significantimprovements are needed. For example, it would be desirable to eliminatethe need for a special cover for shielding specific areas of thesubstrate from the heat generated by the gas jet.

BRIEF SUMMARY OF THE INVENTION

In an embodiment of the present invention a reflow pallet is providedfor soldering electronic components onto a flexible substrate utilizingspecialized cooling arrangements to cool the substrate during the reflowprocess. These cooling arrangements utilize a phase change materialdisposed within internal cavities in the pallet. The phase changematerial absorbs heat from the substrate during a phase transition,thereby maintaining a lowered substrate temperature during reflow. Thisprevents softening of the substrate during reflow, thereby preservingits dimensional stability.

In another embodiment the pallet includes an actuated array ofthermoelectric coolers to cool the pallet. These thermoelectric coolersare actuated as necessary during the reflow process to cool thesubstrate and preserve its dimensional stability. In yet anotherembodiment passages in the pallet through which water, air, or othersuitable fluid is directed to absorb heat from the pallet are provided.These passages keep the substrate cool during the solder reflow process.Thus, the present invention allows the solder reflow of components ontoflexible substrates without the use of a cover to shield the substrateduring the reflow process.

In still another embodiment the pallet and cover are made of a suitableconductive material with good thermal diffusivity, such as a heatresistant carbon fiber composite. Other materials for the pallet includea thin layer of copper backed with a glass-filled epoxy such as FR4.

Preferably, the circuit conductors on the substrate are copper. Selectedregions of the conductors referred to as component pads are providedwith a surface finish such as tin or immersion silver to enhance theease of soldering to the pads. The spaces between the conductor regionsof the substrate may be filled with electrically isolated regions ofcopper having the same thickness as the conductor regions. These copperareas further shield the substrate during reflow by selectivelyabsorbing heat during the reflow process.

Components may be mounted on both the top and bottom sides of thesubstrate. For such a substrate, the reflow process is repeated for thesecond side. The pallet has appropriate cavities to accommodate thecomponents on the first side of the substrate.

The flexible circuit may comprise more than two layers of circuitconductors, commonly referred to as multi-layer circuits. For thesecircuits, two or more layers of the substrate film are used and bondedtogether with a suitable adhesive to form four or more conductor layers.

Any convenient solder paste formulation may be used provided that it canbe activated at a suitable temperature. In an embodiment of the presentinvention a suitable solder paste has a melting temperature of 183degrees centigrade with a composition of 63 percent tin and 37 percentlead. Other solder paste compositions include lead-free solders that arealloys of tin, silver and copper, but exhibit higher meltingtemperatures of about 220 degrees centigrade.

In still another embodiment of the present invention a supplemental heatsource used to activate the solder paste may be supplied by one or morejets of hot gas which are directed toward the exposed areas of thesubstrate. Suitably, the jet of hot gas extends transversely over thewidth of the substrate as it is conveyed past it on a pallet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus for reflowingsolder to electrically connect electronic components to a flexiblesubstrate mounted on a phase-transition pallet, in accordance with thepresent invention;

FIGS. 2 a–2 b is a cross-sectional and plan view of a preferredembodiment of the phase-transition pallet, in accordance with thepresent invention;

FIGS. 3 a–3 d are cross-sectional views of the phase-transition pallethaving a flexible substrate on which electronic components are mountedon both exposed sides of the substrate, in accordance with the presentinvention;

FIGS. 4 a–4 b is a schematic representation of a system for reflowingsolder to electrically connect electronic components to a flexiblesubstrate using a stencil, in accordance with the present invention;

FIG. 5 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a flexible substrate usingan array of hot gas nozzles, in accordance with the present invention;

FIG. 6 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a flexible substrate usingan infra red light source, in accordance with the present invention;

FIGS. 7 a–7 b is a schematic representation of a system for reflowingsolder to electrically connect electronic components to a flexiblesubstrate using a protective cover, in accordance with the presentinvention;

FIG. 8 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a flexible substrate usinga pallet having heat pipes, in accordance with the present invention;and

FIG. 9 is a schematic representation of a system for reflowing solder toelectrically connect electronic components to a flexible substrate usinga pallet having thermoelectric coolers, in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

A system 10 for reflowing solder to electrically interconnect electroniccomponents to a flexible or semi-flexible substrate 12 is illustrated inFIG. 1, in accordance with the present invention. Further, system 10includes a pallet 20 that provides a means to mount circuit componentson flexible substrate 12 without degrading the material properties ofthe substrate. System 10 additionally includes a reflow oven 13, aconveyor system 16, a gas nozzle 18 and a pallet 20. The reflow oven hasa plurality of heaters 22 to pre-heat the substrate 12 to a desiredtemperature. Conveyor system 16 is configured in a conventional mannerto cooperatively receive pallets 14 for movement through the reflow oven13.

Pallet 14 is, preferably, a phase-transition pallet 14 for reflowingsolder paste to interconnect electronic components 24 to flexiblesubstrates 12, in accordance with the present invention.Phase-transition pallet 14 is configured to support substrate 12 andcooperates with conveyor system 16 to transport substrate 12 throughoven 13. Oven 13's heaters 22 pre-heat substrate 12, and hot gas nozzle18 provides supplemental heating. Solder paste 26 is printed onconductor pads 28 disposed on substrate 12 on which components 24 areplaced.

Referring now to FIGS. 2 a–2 b, an elevation and cross-sectional viewsof phase-transition pallet 14 are illustrated, in accordance with thepresent invention. As shown pallet 14 includes at least one internalcavity 40 having therein a phase-change material 42. Support pins 44 areprovided on pallet 14 to hold substrate 12 flat or planar on a palletsurface 46. Pins 44 may be tensioned or loaded by springs 48 to providea tensioning force on substrate 12. In an embodiment of the presentinvention, a picture frame 50 may be used to secure substrate 12 againstpallet surface 46. Picture frame 50, as illustrated attaches to andsecures the periphery of substrate 12 to hold the edges of substrate 12against surface 46 of the pallet.

In another embodiment of the present invention, a phase-transitionpallet 14′ configured to accommodate a double-sided substrate 12 whichelectronic components 24′ are populated on both sides 60, 62 ofsubstrate 12′, is illustrated. In several of the cross-sectional views,as shown in FIGS. 3 a–3 d, pallet 14′ has at least one open cavity 64 toaccommodate electronic components 24′ that have been mounted on thefirst exposed surface 60 of substrate 12′. Open cavity 64 may be filledwith a suitable foam 66, if necessary, to provide additional support forsubstrate 12′.

In a preferred embodiment of the present invention, substrate 12′ is apolyester film having a thickness of 0.003 to 0.010 inches. Copperconductors 68 and solder pads 70 may be formed on both sides 60, 62 ofthe polyester substrate, as is well known in the art. A suitable soldermask (not shown) is applied over copper conductors 68 so that only thepad 70 areas on which solder paste 72 is to be printed are exposed.These pads 70 may have a suitable surface finish such as an organicsurface finish to protect the pad surfaces from oxide formation. Othersurface finishes such as immersion silver or electroplated tin may beused to enhance the solderability of components 24′ to the pads.

Solder pastes 72 that have compositions containing lead, as well assolder pastes having lead-free compositions may be used. The solderpastes containing lead generally have a lower melting temperature ofabout 183° to 200° C., while lead-free solder compositions have meltingtemperatures of about 220° to 245° C.

In operation, as pallet 14 or 14′ having substrate 12 or 12′ affixedthereon is transported through the pre-heat zones in oven, the solderpaste 72 is activated and gradually heated to just below its meltingtemperature. During this process, the phase-transition material 42begins to absorb heat from the oven 13 as well as from the substrate 12or 12′, and thereby lowers the temperature of the substrate. The phasetransition material 42 is selected having a melting point that is lowerthan the melting point of the solder paste 72. As the phase-transitionmaterial 42 begins to melt, the material begins to absorb an amount ofheat or energy equal to the latent heat of the material. Consequently,the temperature of phase-change material 42 is held constant until thematerial is fully melted. Thus, the present invention significantlyenhances the heat absorption properties of the pallet 14 or 14′ andmaintains a lowered substrate 12 or 12′ temperature during reflow of thesolder paste 72.

In a preferred embodiment of the present invention, phase-transitionmaterial 42 exhibits a melting temperature lower than that of solder 72,and may be comprised of conductive metals such as gallium, galliumalloys, or alloys of tin and lead. Other suitable phase transitionmaterials include chloro-fluoro carbons and their compounds.

The supplemental heat created from gas nozzle 18 is utilized to providea focused and concentrated heat source. Gas nozzle 18 provides heat tothe exposed substrate surface for a short duration. The solder paste 26,conductor pads 28, and copper regions of substrate preferable absorbheat because of their high thermal diffusivity, while substrate 12 or12′ is maintained at a lower temperature by the pallet 14 or 14′, whichis held at a lower temperature by the phase-transition material 42. Inthis manner, softening and damage to substrate 12 or 12′ during thereflow process is prevented.

After the exposed region of the substrate passes below gas nozzle 18,the temperature of the exposed electronic component 24 and substrate 12or 12′ rapidly falls so that the activated solder cools and solidifies.A reliable electrical connection between the conductors or pads 20 andcomponents 24 or 24′ is thus formed. During this process, thephase-transition material 42 also solidifies, so that pallet 14 or 14′is ready for reuse.

Referring now to FIGS. 4 a and 4 b, another embodiment of the presentinvention is illustrated wherein a stencil 80 is introduced between thegas nozzle 18′ and the substrate 12 or 12′. The stencil 80 has aplurality of opening or apertures 82 disposed therein. The apertures 82expose certain areas of the substrate 12″ and/or components 24″ to gasnozzle 18′ to reflow the solder paste 72′. The stencil 80 also shieldssubstrate 12″ areas and/or components that are not to be exposed to thegas jet In this manner solder paste is melted in the appropriate areasand potential damage caused by heating the substrate to elevatedtemperatures is avoided. In another embodiment as shown in FIG. 4 a, thepallet 14″ and stencil 80 are held stationary while the gas nozzletraverses the stencil to selectively heat the substrate areas. Inanother embodiment, as shown in FIG. 4 b, a hot gas nozzle 18″ is heldstationary while pallet 14′″ and stencil 80 move below the gas nozzle18″. This embodiment would require multiple stencils to heat and reflowthe desired areas of the substrate and electronic components.

In yet another embodiment of the present invention, a system 90 forreflowing solder is illustrated in FIG. 5. The present embodimentcontemplates system 90, having an array of gas nozzles 92 positionedabove a conveyor system 94. The array of gas nozzles 92 are computercontrolled and as such may be programmed to separately actuate for adefined period of time. The gas nozzles 92 are programmed to actuate andrelease high temperature gas on selected areas of a populated substrate96 to reflow the solder paste as the components 98 pass underneath thearray of nozzles 91. Preferably, a downward facing camera 100 or opticalscanner is used to read a bar code 102 printed on substrate 96 toidentify the substrate 96 and program the actuation of the array of gasnozzles 91. Array 91 may be constructed from a silicon micromachinedstructure and contain silicon micromachined valves. Other selectiveheating techniques such as soft beam may be used wit the gas nozzlearray 91. Moreover, the present invention contemplates using differentgas pressures in different gas nozzles in the array 91.

In still another embodiment of the present invention, as shown in FIG.6, a system 149 for reflowing solder using an infra-red light source 150as a supplemental heat source 152 is illustrated. In the instantembodiment a substrate 152 is covered with a protective cover 154 thatis impervious to infra-red radiation. Protective cover 154 has aplurality of apertures 156 for exposing the electronic components 158 tobe soldered to substrate 152. The infrared light source 156 may includea plurality of infra red devices to produce a desired heating effect.Further, a Collimating lens 160 is placed between the infra red light150 and the populated substrate 152 to focus the light directed towardthe substrate. Once the protective cover 154 is in place the pallet162/substrate 152/cover 154 assembly is placed on conveyor 16 andtransported through reflow oven 13. The temperature of the oven 13 maybe set at a temperature that does not damage flexible substrate 152. Theadditional heat energy required to reflow the solder paste, disposedbetween the electronic components 158 and the solder pads on thesubstrate is supplied by the infra red light source 150.

FIGS. 7 a–7 b illustrates a protective cover 200 for shielding portionsof the substrate from hot gasses emanating from gas nozzle 202. In anembodiment of the present invention protective cover 200 is made frominsulative materials such as FR4 material or aluminum or the like. Thistype of cover may combined with any of the previous embodiments, asappropriate.

FIG. 8 is a cross-sectional view through a pallet 300. Pallet 300supports a flexible substrate 302 populated with electronic components304. Pallet 300 includes a plurality of heat pipes 306 which draw heataway from substrate 302 to cooler regions of the pallet. Additionally,the heat pipes are in communication with phase transition regions 310which contain phase transition material, as described previously. Theheat pipes and phase transition regions 310 cooperate to cool thesubstrate 302 to insure the substrate is not damage by exposure to thesupplement a heat source.

In another embodiment of the present invention, as shown in FIGS. 9 a–9b, a pallet 400 is provided having thermoelectric coolers 403 to absorbheat away from substrate 402. As in the previous embodiments asupplemental heat source is applied to substrate 402 populated withelectronic components 404 to reflow solder disposed there between. Asshown in FIG. 9 a, the present invention contemplates an array 408 ofthermoelectric coolers 403 disposed in pallet 400. The array 408 may beindependently actuated and controlled to provide localized cooling.

While the present invention has been particularly described in terms ofthe specific embodiments thereof, it will be understood that numerousvariations of the invention are within the skill of the art and yet arewithin the teachings of the technology and the invention herein.Accordingly the present invention is to be broadly construed and limitedonly by scope ad spirit of the following claims.

1. A system for reflowing solder to interconnect a plurality ofelectronic components to a substrate, the system comprising: an oven forpreheating the substrate and the plurality of electronic componentsdisposed thereon; a supplemental heat source disposed in the oven forproviding additional heat energy to reflow the solder; a pallet forsupporting the substrate, wherein the pallet has an internal cavityhaving a phase-transition material disposed therein for absorbing heatfrom the substrate; and a stencil positioned between the supplementalheat source and the electronic components to selectively direct heatfrom the supplemental heat source onto predefined portions of thesubstrate.
 2. The system of claim 1 wherein the pallet has at least onetensioned pin to hold the substrate flat against the pallet.
 3. Thesystem of claim 1 further comprising a cover for covering portions ofthe substrate not to be exposed to the supplemental heat source.
 4. Thesystem of claim 1 wherein the phase-transition material is an alloycontaining tin and lead.
 5. The system of claim 1 wherein thephase-transition material is an alloy containing gallium.
 6. The systemof claim 1 wherein the supplemental heat source is a gas nozzle fordirecting hot gas onto the substrate.
 7. The system of claim 1 furthercomprising a conveyor for transporting the pallet through the oven andunder the supplemental heat source.
 8. The system of claim 1 wherein thepallet further comprises an open cavity to accommodate electroniccomponents mounted to both sides of the substrate.
 9. The system ofclaim 1 wherein the open cavity further comprises a foam to providesupport for the substrate.
 10. The system of claim 1 further comprisinga stencil positioned between the supplemental heat source and theelectronic components to direct hot gas onto portions of the substrate.11. The system of claim 1 wherein the supplemental heat source furthercomprises an array of hot gas nozzles, wherein the nozzles areindependently actuatable and controllable to reflow the solder.
 12. Thesystem of claim 1 wherein the supplemental heat source is an infra redlight source.
 13. The system of claim 12 wherein the infra red lightsource further comprises a collimating lens for directing the infra redlight onto the electronic components.
 14. The system of claim 1 whereinthe pallet further comprises at least one thermoelectric cooler forabsorbing heat from the substrate.
 15. The system of claim 1 wherein thepallet further comprises at least one heat pipe for absorbing heat fromthe substrate.
 16. The system of claim 1 wherein the pallet has at leastone internal cavity.
 17. The system of claim 16 further comprising aphase-transition material disposed within the cavity for absorbing heatfrom the pallet.